Mesogenic Groups In Side Chains Research Articles

Dielectric and thermal behavior of liquid crystalline comb-like polybutadiene-diols with mesogenic groups in side chains

Ordered polybutadiene-diols (LCPBDs) with the comb-like architecture were prepared by radical reaction of a 5-(4-{[4-(octyloxy)phenyl]azo}phenoxy)pentane-1-thiol with double bonds of telechelic HO-terminated polybutadiene (PBD); several polymers with various initial molar ratios of thiol to double bonds of PBD, R0, in the range from 0 to 1 were prepared. DSC, polarizing microscopy, WAXS and dielectric relaxation spectroscopy (DRS) were employed to investigate their thermal and dielectric behavior in relation to morphology. DRS of the LCPBDs have revealed both collective and individual dynamic motions of molecules. Secondary β- and segmental α-relaxation were observed in unmodified PBD. In the LCPBDs, two secondary γ- and β- and two high-temperature α- and δ-relaxations were observed and assigned to specific molecular motions; all relaxations were analyzed and discussed in terms of time scale (Arrhenius diagram), magnitude (relaxation strength) and shape of the response.

Dynamic mechanical and thermal behavior of liquid-crystalline polybutadiene-diols with mesogenic groups in side chains

Liquid-crystalline polybutadiene-diols (LCPBDs) with the comb-like architecture were synthesized by reaction of a LC thiol with the double bonds of telechelic HO-terminated polybutadiene (PBD). LCPBDs with various initial molar ratios of thiol to double bonds of PBD, R 0, in the range from 0.15 to 1, were prepared by the radical reaction at temperature 60 °C for 48 h. The experimentally obtained degree of modification, R e, after the reaction and purification, was determined from elemental analysis – from the amount of sulphur bounded in LCPBDs, GPC and from 1H NMR spectra. The physical properties were investigated by differential scanning calorimetry and dynamic mechanical spectroscopy. With increasing R e ratio the glass transition temperature of LCPBDs, T g, increases from ∼ − 45 °C (neat PBD) to ∼20 °C ( R e ∼ 0.5). LC transition starts at R e ∼ 0.27 (the transition temperature T m ∼ 27 °C). With increasing R e temperature T m increases and for R e ∼ 0.5 reaches the value T m ∼ 74 °C; at the same time also the change in enthalpy at LC transition increases. The LC transition could be detected also by the dynamic mechanical spectroscopy; especially shape and position of mechanical functions on frequency and free volume parameters strongly depend on degree of modification.

Characterization of molecular structures and properties of polyurethanes using molecular dynamics simulations.

Thermotropic polyurethanes with mesogenic groups in side chains were prepared from two diisocyanates and four diols with stoichiometric ratios of reactive isocyanate (NCO) and hydroxy (OH) groups. Their thermal behavior was determined by differential scanning calorimetry. The effect of structure modifications of the diisocyanates and diols, in particular changes in the mesogen, were investigated. Introduction of mesogenic segments into the polymers suppresses the ordering. Stiff end substituents (phenyl and alkoxy groups) of the mesogens stabilize the mesophases to such an extent that the negative influence of long polymer chains is compensated and the liquid-crystalline properties are recovered. All-atom molecular dynamics simulations in the Cerius2 modeling environment were carried out to characterize the structures of the polymers. Analysis of the dynamic trajectories at 20, 100, 120 and 170 degrees C revealed changes in conformation of macromolecules, which correlate with DSC measurements.

Thermal and dynamic mechanical behavior of polyurethanes based on diisocyanates and diethanolamine derivatives with mesogenic groups in side chain

Thermotropic polyurethanes were prepared from three commercial diisocyanates of various flexibility and eight diols (based on diethanolamine derivatives) with mesogenic groups in side chain with stoichiometric ratio of reactive isocyanate and hydroxy groups. Polymers were studied by dynamic mechanical spectroscopy, X-ray scattering, differential scanning calorimetry and polarizing microscopy. The effect of structure changes in the diisocyanates and diols, in particular changes in the end substituents bound to the mesogen, were investigated. Introduction of mesogenic diols into the polymers suppresses the occurrence of mesophases in comparison with neat diols; in the case of simple end substituents (such as hydrogen, nitro and nitrile), the mesophases disappear completely regardless of the structure of diisocyanate. Stiff end substituents (phenyl and alkoxy groups) stabilize the mesophases to such an extent that the negative influence of long polymer chains is compensated and the liquid–crystalline (LC) properties are recovered. Generally, the polymers prepared from the stiffest 2(4)-methyl-1,3-phenylene diisocyanate exhibit a most pronounced LC behavior.

Structural transformations in polymer macromolecules with mesogenic groups in side chains

Polarization luminescence, viscometry and spectropolarimetry were used to study structural transformations in cholesteric ester macromolecules of polymethacroyl-ω-hydroxycarboxylic acids in polar and nonpolar solvents in a wide range of temperature. It was found that in macromolecules of these polymers under certain conditions nuclei of mesophase are formed, the interaction of which results in a conformation transition of sphere-globule type. The importance of the kinetic factor in forming nuclei of the mesophase was demonstrated. It was shown that the length of side chains-carriers of mesogenic groups and solvent viscosity influence the formation of globular structures. A study was made of the effect of the molecular weight of cholesteric esters of polymethacryloyl-ω-hydroxycarboxylic acids on parameters reflecting structural transformations.